1. Technical Field
This invention relates generally to construction equipment, and more particularly to a machine for laying or placing a concrete slab where there is little or no side clearance.
2. Background Information
A paving machine or slipform paver, such as those commonly used to place concrete highways, includes a pan assembly that is supported by a frame structure on a tractor or other prime mover. As the tractor inches forward along a roadway to be paved, the left and right tracks straddle a supply of concrete dumped onto the roadway in a hopper arrangement on the front of the tractor and one or more augers move the concrete transversely across the pan assembly. There, an extrusion-process pan on the pan assembly rides over the concrete, working it into a finished slab while sensor-controlled hydraulics attempt to maintain just the right slab elevation or thickness. This is done to place many miles of pavement so that the details of paving machine construction are of interest.
The pan has a downwardly facing planar surface that extends laterally across the width of the slab and rearwardly from a leading edge that is disposed toward the hopper to a trailing edge. The leading edge is slightly higher than the trailing edge so that the pan extrudes the concrete rearwardly. In this regard, the pan can be thought of as working the concrete somewhat like a trowel.
One problem with some existing paving machines concerns operation of the pan when the roadway to be paved has little or no clearance along one or both of its sides. Such minimum or zero clearance paving may occur, for example, when the roadway to be paved provides a new traffic lane alongside an existing traffic lane or extends to curbing that has already been poured. In such cases, the roadway extends up to an existing structure so that the new pavement will abut the existing structure.
Thus, there is little or no clearance on the sides, and this presents a problem because the left and right tracks must ride on the roadway to be paved. With the tracks riding on the roadway, the pan, which lies in between the tracks, cannot cover the full width of the roadway. Thus, unpaved strips remain, one behind each of the left and right tracks.
Some existing paving machines include left and right pan extensions behind the tracks in order to solve this problem, along with an auger system that employs five augers to move the concrete rearwardly to the pan extensions. Other existing paving machines designed for one-sided minimum clearance applications employ a four-track tractor having the rearward track removed on the minimum clearance side and a pan extension installed in the area of the removed track. This enables the pan to be extended behind the front track on that one side.
In either case, a pan extension extends behind at least one of the left and right tracks and this introduces certain other problems. In particular, and apart from being somewhat inconvenient and complicated, it may cause poor elevation control. In other words, control of pan position may be affected so that the elevation of the slab is difficult to control.
Consider, for example, the sensor-controlled hydraulics of an existing paving machine. They may include one or more sensors mounted on the frame structure, each having a wand or sensor arm that rides on a wire strung alongside and at a known elevation above the roadway to be paved (sometimes referred to as a null position). Electrical control circuitry responsive to signals produced by the sensors causes hydraulic cylinders to adjust the height of the frame structure and thus the pan until the arm of each sensor is riding on the wire at the null position (causing what is sometimes called a null condition). This is done in an effort to maintain the pan at the desired elevation.
However, when pan extensions are added behind the tracks, sensor arm height does not accurately reflect the height of the pan extensions. In other words, the pan extensions can move vertically a significant distance without the sensor arm moving the same amount because of the lever arm between the sensor arm and the rearwardly disposed pan extensions. This is aggravated by the tendency of the pan extensions to float on the concrete mud and the tendency for mud consistency to vary from batch to batch. Poor elevation control results.
Consequences of poor elevation control may be severe and include the cost of unneeded concrete and the need to grind down significant stretches of new pavement in order to blend into the existing grade. Thus, it is desirable to have a new and improved paving machine that overcomes the above concerns.
Another problem with some existing paving machines concerns obstacles along the roadway. Obstacles such as sewage and drainage structures may exist along the roadway in the path of the tractor and with existing paving machines it may be necessary to stop paving, veer around the obstacle, and then resume paving on the other side of it. This is inconvenient and it adds cost to the project. Thus, it is desirable to have a paving machine that alleviates this concern as well.